Natural gas typically contains up to 15 vol % of hydrocarbons heavier than methane which are separated to provide pipeline quality methane and recovered liquid hydrocarbons. These valuable natural gas liquids comprise primarily ethane, propane, butane, and minor amounts of other light hydrocarbons which are recovered from natural gas following compression and initial removal of nonhydrocarbon acid gases, water, and other impurities.
Natural gas liquids (NGLs) are recovered from compressed prepurified natural gas by well-known methods including absorption, refrigerated absorption, adsorption, and condensation at cryogenic temperatures down to about -175.degree. F. Refrigeration for low temperature recovery processes is commonly provided by external refrigeration systems using ethane or propane as refrigerants; mixed refrigerants and cascade refrigeration cycles are used in some applications. Refrigeration also is provided by turboexpansion or work expansion of the compressed natural gas feed with appropriate integral heat exchange.
A general review of NGL recovery methods is given by C. Collins, R. J. J. Chen, and D. G. Elliot in a paper entitled "Trends in NGL Recovery from Natural and Associated Gases" in Gastech, Gastech LNG/LPG Conference 84, Published by Gastech Ltd., Rickmansworth, England, pp. 287-303. The use of mixed refrigerants in NGL recovery is reviewed in a paper entitled "Mixed Refrigerants Proven Efficient in Natural-Gas-Liquids Recovery Process" by D. H. MacKenzie and D. T. Connely in Oil and Gas Journal, Mar. 4, 1985, pp. 116-120. J. D. Wilkinson and H. M. Hudson disclose turboexpander cycles for NGL recovery in a paper entitled "Improved NGL Recovery Designs Maximize Operating Flexibility and Product Recoveries" in the Proceedings of the 71st GPA Annual Convention. Refrigeration for NGL recovery by these well-known methods is provided either by external refrigeration or by work expansion of the compressed natural gas feed.
An alternative and improved method for NGL recovery which does not require external refrigeration or work expansion of the feed gas has been developed in the present invention as described in the specification below and defined in the claims which follow.